The present invention relates generally to three-dimensional display of information in an optically transparent solid medium that is doped with at least one active ion or molecule to provide conversion of infrared radiation to visible light by means of a two-photon upconversion process. The invention also relates to the use of laser diodes, laser diode arrays, and tunable solid-state lasers as sources of the infrared pumping radiation.
Two-photon absorption is a well known process in which two distinct photons of the same or different energies are absorbed by an ion or molecule, causing excitation from the ground state to a higher energy state to be achieved. The excitation pathway can involve either a real or virtual intermediate energy state, with the former case also referred to as resonant two-step absorption. In either case, the ion or molecule remains in the upper excited state for a short time, commonly known as the excited state lifetime, after which it relaxes back to the ground state, giving up the excess energy in the form of phonons (referred to as nonradiative relaxation) or photons (referred to as radiative relaxation, leading to upconversion fluorescence or possibly stimulated emission). When considering fluorescence, an important figure of merit is the quantum efficiency, defined to be the visible fluorescence intensity divided by the total input pumping intensity.
For display applications based on two-photon induced fluorescence, radiative relaxation must dominate over nonradiative relaxation in order to obtain a sufficient quantum efficiency, and moreover, the two infrared photons must derive from separate sources with distinctly different wavelengths so that visible fluorescence occurs only in those regions of the host material where the beams from the separate sources overlap. In this way a true three-dimensional image or sequence of images can be drawn in the material by rapidly scanning or otherwise manipulating the pump beams, which are invisible to the eye, so as to activate the appropriate voxels comprising the images. To ensure that visible fluorescence is generated only in the region of pump beam overlap, single-frequency upconversion arising from two-photon absorption of the individual infrared beams must be minimized; otherwise, visible "streaking" of the pump beams will be apparent.
Earlier efforts to develop volumetric 3-D displays based on two-photon induced fluorescence were hindered by the lack of transparent host materials exhibiting high quantum efficiencies, and by the lack of convenient laser pump sources at the excitation wavelengths of the active ions. In 1963, R. Zito proposed a display in mercury vapor that required pump sources at 250 nm and 400 nm (Appl. Phys., Vol 34, 1963, pg 1535), but this approach was limited in a number of important ways including the need for intense lasers in the ultraviolet and blue regions of the spectrum as well as the problems associated with containing a large volume of toxic mercury. For these and other reasons, the Zito display was not pursued beyond the point of conceptualization.
An upconversion display using infrared excitation sources in Er-doped CaF.sub.2, described by J. D. Lewis et al. in a 1971 publication (IEEE Trans. Electron Dev., Vol ED-18, 1971, pp 724-732) and in U.S. Pat. No. 3,829,838 entitled "Computer-Controlled Three-Dimensional Pattern Generator," suffers from low quantum efficiency and weak fluorescence inherent to the use of CaF.sub.2 as a host material. The poor performance of this host is related to its high phonon energy which promotes radiationless relaxation via phonon production. In addition, the use of Er.sup.3+ as the active ion is now known to be a poor choice for upconversion display because it possesses a preponderance of closely spaced spin-allowed transitions, making single-frequency upconversion, and its concomitant streaking effect, a significant problem. More specifically, when each of the individual infrared pumping wavelengths cited in their patent (namely 1.53, 1.14, 0.84 and 0.79 .mu.m) is focused separately into Er.sup.3+ :CaF.sub.2, green light is readily generated along most of the beam path. The problem of addressing the issue of single-frequency upconversion and its deleterious effect on the performance of the display is strikingly absent from the prior art. Lastly, trivalent erbium has a multitude of closely spaced energy levels located above those from which visible light is emitted. These higher levels offer pathways for depletion of the desired levels through additional excited-state absorption, thereby effectively reducing the quantum efficiency.
Recent research into heavy-metal fluoride glasses has yielded many new rare-earth-doped glasses that are also well suited for use in two-photon 3-D display applications. This materials research has been motivated largely by the need for efficient fiber amplifiers (M. A. Newhouse et al., IEEE Phot. Tech. Lett., Vol. 6, 1994, p. 189) and for fiber lasers (A. C. Tropper et al., J. Opt. Soc. Am. B, Vol. 11, 1994, p. 886) in the telecommunication industry, and by the need for short wavelength lasers for high-density optical data storage (W. J. Kozlovsky et al., Proc. Soc. Instr. Eng., Vol. 1663, 1992, p. 410). The development of volumetric 3-D display as disclosed in this invention is further facilitated by the commercial availability of high-power infrared laser diodes, laser diode arrays, and tunable solid-state infrared lasers.
There exists a multitude of applications for a solid-state 3-D display system that does not require special viewing devices (e.g., glasses, headgear) and offers viewing access from a wide range of positions with minimal restrictions. For example, air traffic control and other radar-based technologies could effectively utilize such a system to display three-dimensional scenes in real time. Another major application area includes medical imaging, in which highly developed three-dimensional data acquisition systems based on ultra-sound, magnetic resonant imaging, and cat-scans are currently being used. Engineering work stations and computational finite element analysis packages would also benefit since data that has three spatial dimensions often has temporal variations in other parameters like pressure, temperature, stress, velocity, etc., which can be displayed by means of dynamic 3-D color images. Entertainment in the form of video games and 3-D movies is yet another area of significant opportunity for this technology.